Datasheet

Application information L6563 - L6563A
20/39
The twice-mains-frequency (2·f
L
) ripple appearing across C
FF
is triangular with a peak-to-
peak amplitude that, with good approximation, is given by:
Equation 4
where f
L
is the line frequency. The amount of 3
rd
harmonic distortion introduced by this
ripple, related to the amplitude of its 2·f
L
component, will be:
Equation 5
Figure 36 shows a diagram that helps choose the time constant R
FF
·C
FF
based on the
amount of maximum desired 3
rd
harmonic distortion. Always connect R
FF
and C
FF
to the
pin, the IC will not work properly if the pin is either left floating or connected directly to
ground.
Figure 36. R
FF
·C
FF
as a function of 3
rd
harmonic distortion introduced in the input
current
The dynamics of the voltage feedforward input is limited downwards at 0.5V (see Figure 35),
that is the output of the multiplier will not increase any more if the voltage on the V
FF
pin is
below 0.5V. This helps to prevent excessive power flow when the line voltage is lower than
the minimum specified value (brownout conditions).
V
FF
∆
2V
MULTpk
14f
L
R
FF
C
FF
+
---------------------------------------=
D
3
%
100
2πf
L
R
FF
C
FF
---------------------------------=
D %
3
0.1 1 10
0.01
0.1
1
10
f = 50 Hz
L
f = 60 Hz
L
R · C [s]
FFFF